Transmitting circuit



1935. c. A. BODDIE TRANSMITTING CIRCUIT Filed Sept. 16,

INVENTOR C/a rence A. Bodalle ATTCsRNEY TNESSES:

Patented y 14,1935 Q 1 accuse sm'rmo cmcnrr Clarence a. Boddie, burg, p

to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania Application September 16, 1926, Serial No. 135,791 7 Claims. (Cl. 177-352) My invention relates to transmitting circuits, mentalities involved may. be had, attention is and it has particular relation to circuits for'use directed to the accompanying drawing, and to in connection with the transmission of carrier the following description, wherein currents along metallic channels, such as power Figure 1 is a. perspective view of one of the lines, trolley wires and the like. 1 power-line supp n P h w he Spa 5 One object or my invention is to provide means relationship existing between the power lines and whereby high-frequency currents for transmitthe coupling antenna. ting intelligence may be successfully carried Fig. 2 is a diagrammatic view of the circuits across .breaks in a transmission line, involved in a preferred form of my invention,

1 Another object of my invention is to provide 3 is a diagrammatic view of an optional means whereby carrier currents may be byform of my invention, passed around transformer banks and similar ap- Fig. 4 is a dia ram u ra h r i n paratus. a I relations between the various types of tuned cir- Another object of my invention is to provid cults embodying my invention,

in systems of the type described, means whereby Referring p fi y t0 Fig-' a plurality 0 15 high-frequency currentsrepresenting messages power l nes 2 and 3 are supp d n spaced may be carried around a break in a transmisati n y 9 4 n cross-arm 5 amxed sion line without any great loss or diminution in thereto. A second cross-arm 8 is suppo ted on am litude, the pole 4 and carries, at its extremities, a

1- Still another and more specificobject of my p rality oi coup n antenna: .1 and 8. A single invention is to provide, in carrier-current sys- P o ly is h wn n h wi i being 11ntems, means whereby the capacitive impedance derstood, howev r, h a plur y 01 90188 would of a break in the transmission line may be com vbe required in actual practice to support an adepensated. quate length of antenna.

- The successful application of carrier-current In P18. 2, a pl r y D wer-lines I, 3 and '3 transmission along power lines has been very have their electrical continuity interrupted bya seriously handicapped by the presence in such series of gap r i nnec Swi ches "I. H and lines of transformer banks of very high im- '2, or other apparatus offering high impedance pedance to the frequencies used for the trans-. to the carrier-frequencies employed for transmitmission of intelligence. The necessity for the ting intelligence, An antenna I3 is capacitively 30 interposition of circuit breakers, disconnect pled to one of the power lines and bridges the switches and other circuit-interrupting devices gap formed by the disconnect switch It. Asecond has also proved detrimentaL. antenna I4 is capacitively coupled to another of In order that there shall be no interruption the power lines 8, bridging the gap across the disto the transmission of messages, it is necessary connect switch I 2. Between these two antennae 35 that all unavoidable breaks in the metallic 0011- I3 and I4 is connected a circuit comprising an inductor shall be bridged or lay-passed by instruductor IS, an inductor l8 shunted by a condenser mentalities capable of efliciently transferring the ii to form a parallel resonant circuit, and an carrier current from one side of the break to the other inductor i8.

40 other. Although I have shown three power'lines for 40 It is not feasible to merely couple a conductor the transmission of polyphase currents, only two capacitively to the power line at each side of the of such lines are utilized for signaling purposes. break, depending on the capacity between the Ihave accordingly illustrated myinvention as apconductor and the line to carry the high-.freplicable'to only two of the three. lines, it being quency current, as the impedance of the conobvious that it is also applicable'to power sys- 4 denser formed between the line and conductor tems employing only two wires.

would be unduly high. I have found, however, The entire circuit, comprising the antennaethat, by making use of the correct amount of inpower-line capacities. and the various inductdu'ctance, the capacity between the line and the ances, is tuned in a definite manner, which will coupling conductor may be made use of to tune be explained later.

the coupling in such manner as to obtain sub- At the present time, it is customary to employ stantially zero reactance from line to line, at two diflerent high-frequencies in carrier-curthe frequencies used. rent communication over power lines, one freln order that a complete understandingof the quency being used for sending and the other for theory underlying my invention, and the instrureceiving, both frequenciesbeing modulated by the signals being transmitted. If there happen to be open disconnect switches in the power lines,

some manner.

or a transformer ofiering high impedance to the carrier frequencies, considerable dimculty would be experienced in efliciently by-passing the highfrequency currents. It is, accordingly, necessary that the gaps, or the transformer, offering high impedance to the -carrier current, be bridged in Obviously, the gaps cannot be bridged by metallic connections, as such connectionswould defeat the purpose of the gaps, yet, at the same time, it is imperative to use bridging means offering low impedance to the carrier currents.

The natural and obvious solution of the difficulty would, at first glance, seem to be merely the use of bridging antennae, capacitively coupled to the power lines at each side of the breaks. In view, however, of the relatively high voltages now being used in power transmission, the coupling antennae would have to be spaced well away from the power lines, and the capacitive impedance of this space would be prohibitively high.

By my invention, I provide means for nullifying the capacitive impedance of the space between the power lines and the coupling antennae, eifectively, tuning the impedance to the frequencies employed.

Insofar as the carrier currents are concerned, the two sections into which the power lines are cut by the disconnect switches may be considered as having negligible inductance and capacity, the total line impedance at each side of the break being merely resistive in character. In the usual type of power circuit, the line at each side of the break may be considered as replaced by bridging resistors 25 and 26 of from 600 to 800 ohms. Inasmuch as the carrier currents in each line are, at all times, opposite in phase with respect to ground, the line, at each side of the break, may be considered simply as an open circuit on which is impressed an alternating potential and having in series therein a high resistance. The problem, therefore, resolves itself into one of providing a transformer between the two circuits so cons'tructed as to transfer high-frequency energy from the one to the other without unduly high losses.

It is well knownthat, when, for a given frequency, the capacitive reactance is equal and opposite to the inductive reactance in a series circuit, the total impedance, neglecting the resistance, is zero. It is also known that, under the same conditions, a parallel circuit offers infinite impedance to currents of the critical or resonant frequency. At frequencies higher than the resonant frequency, the reactance of the series circuit is predominately, inductive, while, at lower frequencies, the reactance is mainly capacitive.

In Figure 4, is shown a set of curves illustrating the manner in which the reactance of series tuned and parallel tuned circuits varies with the frequency, An axis :r-m is chosen to represent frequencies, those below the resonant frequency being laid off to the left of a vertical axis y-y, while those above the resonant frequency are represented by distances to the right of the y-y axis. Distances along the 11-11 axis above the a:-a: axis represent inductive reactance, and those variation in reactance with frequency for a series tuned circuit, the curve crossing the :v-a: axis at the resonant frequency. The curves (3-H and IJ represent the resultant reactance of a series-tuned circuit in series with a parallel resonant circuit, and it willbe noted that there are two points, K and L, respectively, below and above the resonant frequency, at which the total reactance of the circuits is zero.

If the inductors l5 and H! are now properly chosen with reference to the antennaa-to-powerline capacities to form a series tuned circuit resonant to a frequency half way between the two frequencies used for transmission and reception,

complex circuit functions. It will help, however,

to state that the coils I 5, l5 and 18 function as aone-to-one-ratio auto-transformer, and as such,

comprise means whereby the signal frequencies may be transferred from one side of the break in the line to the other. By reason of the small capacity existing in the coupling between the antennze and the power lines, the power frequency is efiiciently excluded from this by-pass circuit.

In the event that only a single frequency is employed for both sending and receiving, it is not necessary to provide a by-pass circuit resonant to two frequencies. In the arrangement shown in Fig. 3, a plurality of power lines I, 2 and 3 have their electrical continuity broken by a plurality of disconnect switches III, II and i2. Anantenna I9 is capacitively connected to one of the power lines I and is conductively connected through an inductor 2| to the same line I at the other side of the break If). A second antenna 22 is capacitively connected to another power-line 3, and is also conductively connected through an inductor 23 to the line 3 on the other side of the break l2 therein.

Again, as explained in connection with Fig. 2, the line on one side of the break may be replaced by ,an equivalent resistance 25, and the line on the other side of the break by an equivalent resistance 26. The circuit comprising the inductor 2| the capacity between the antenna l9 and the power line I, the resistor, the capacity between the other power line 3 and the antenna 22, and the second resistor 26, is tuned to the carrier-current frequency by means of the variable inductors 21 and 23. The power line and antenna, being then series-resonant to the signalling frequency, offer substantially no impedance to currents at that frequency, permitting maximum energy interchange between the sections of the line separated by the disconnect switches.

My system thus provides an extremely eflicient circuit for by-passing high-frequency carriercurrents either around a physical break in the power lines or around apparatus which normally offers very high impedance to the frequencies used for signalling. By reason of the fact that my tuning arrangement provides so efficient a path for high-frequency-energy interchange between the power lines and the coupling antenna,

it, is unnecessaryto increase the coupling capacity to the point where there is danger from arc-overs and short-circuits. The power necessary for efficient signalling may be greatly reduced from that required when simple by-pass means are used, with a consequent reduction both in the first cost of the transmitting equipment and in the cost of upkeep.

While I have shown and described only a few specific embodiments of my invention, it is apparent that various changes and modifications may be made in the nature and details of arrangement, without departing from the spirit thereof. I desire, therefore, that only such limitations shall be imposed thereon as are necessitated by the prior art or as are indicated by the appended claims.

I claim as my invention:

1. In a power system employing a plurality of sources of signal energy at different signalling frequencies for duplex communication thereover, a' plurality of conductors, apparatus interposed between said conductors of a type through which the signalling energy cannot be transmitted and means for by-passing said signalling energy about said apparatus, said means comprising coupling means between said conductors of a type having the property of an electrical impedance that is large for the frequencies of said signalling energy and a plurality of electrical impedances coupled to said coupling means and forming with said coupling means a network of resonant and anti-resonant circuits which network is resonant at a plurality of the frequencies of said signalling energy.

2. In a power system employing a plurality of sources of signal energy at diiferent signalling frequencies for duplex communication thereover, a plurality of conductors, apparatus interposed between said conductors of a type through which i the signalling energy cannot be transmitted and means for by-passing said signalling energy about said apparatus, said means comprising coupling means between said conductors of a type having the property of an electrical impedance that is large for the frequencies of said signalling energy and a plurality of electrical impedances coupled to said coupling means and forming with said coupling means a network consisting of a resonant circuit in series with an anti-resonant circuit, which network is resonant at a plurality of the frequencies of said signalling energy.

3. In a power system employing a plurality of sources of signal energy at different signalling frequencies forduplex communication thereover, a plurality of conductors, apparatusinterposed between said conductors of a type through whichthe signalling energy cannot be transmitted and means for by-passing said signalling energy about said apparatus, said means comprising coupling means between said conductors of a type having the property of an electrical capacity,"the impedance of which is large for the frequencies of said signalling energy and a pluralityof inductors and condensers coupled to said coupling means and forming with the capacity of said coupling actance elements comprising a reactance element of opposite sign to the first-mentioned reactance element in series with two reactances of opposite sign connected in multiple with each other, the two last-mentioned reactances bein tuned to constitute an anti-resonant circuit at the same frequency at which the first-mentioned and second-mentioned reactances constitute a series-resonant circuit. I

5. In combination with a power transmission line having at least two line wires, sources of signal energy of two different frequencies for duplex communication thereover, an apparatus of a type interposing a high impedance to said frequencies in each said line wire and means for by-passing said frequencies about said apparatus, said means comprising a reactance element bridging the terminals of each said apparatus, and a connection between said two reactance elements comprising a reactance element of opposite sign to said first-mentioned reactance element in series with two reactances of opposite sign connected in multiple with each other, the two last- 'mentioned reactances being tuned to constitute an anti-resonant circuit at the same frequency at which the first-mentioned and second-mentioned reactances constitute a series-resonant circuit, said resonant frequency lying halfway between the two frequencies first mentioned.

6. In combination with a power transmission line having at leastv two line wires, sources of signal energy of two different frequencies for duplex communication thereover, an apparatus'of a typefi interposing a high impedance to said frequencies in each said line wire and means for by-passing said frequencies about said apparatus, said means comprising a conductor for each said apparatus capacitatively coupled to the line wires atfeach terminal of said apparatus, a connectionbetween said conductors comprising inductance in series with an anti-resonant circuit, said inductance and said capacitative couplings being in series resonance at the same frequency at which said anti-.-

resonant circuit is tuned.

'7. In combination with a pair of line wires in series with means for imposing two different frequencies and having interposed between them a high impedance to said frequencies, a channel bridging said impedance comprising a capacitance and an inductance in series with each other and with an anti-resonant circuit consisting of a capacitance and an inductance in parallel.

l CLARENCE A. BODDIE. 

